Densitometer



`March 9, 1948. c. M. TUTTLE DENSITOMETER Filed Nov. 8, 1945 y2 Sheets-Sheet 1 CLIFTON M. TUTTLE `INVENTOR vBY :im @hay ATTORNEY March 9, 1948.

c. M. TUTTLE 2,437,411

DENS ITOME'IER4 Filed Nov..'8, '1945 2 sheets-shet 2 CLIFTON M. TUTTLE INVENT OR M A TTORNE YS Patented Mar. y9, i948 DENSITOMETER Clifton M. Tuttle, Rochester, N. Y., alaignor to Eastman Kodak Company, Roch corporation of New Jersey eater, N. Y., a

Application November s, '1945. serial Nes'zmsvs y18 Clalml. (Cl. 88-3-14) 'I'he present linvention relates to a densitometer.

Y and particularly to a densitometer of the transmission type to be used for measuring the density of photographic film.

The primary object of the present invention is to provide a low priced densitometer which is not only pleasing in appearance and easy to operate, but which is very efficient in operation.

Another object is to provide a densitometer which eliminates the use of all expensive optical elements and which can be fabricated. for the most part, from plastics by molding.

And another objectl is to provide a densitometer in which two light transmitting and integrating rods of light transparent material (plastic) transmit the light passing through the nlm and that passing through the density wedge to a point for comparisomthe exit ends of the two rods constituting, in themselves, the comparison or,k

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation. together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings in which,

Fig. 1 is a top plan view. partly in section, of a densitometer constructed in accordance with a preferred embodiment of the present invention,

Fig. 2 is a front view, partly in section, of the densitometer,

Fig. 3 is a vertical section taken substantially on line 3-3 of Fig. 1,

Fig. 4 is an enlarged sectional detail showing the form of the entrance end of the light transmitting rod for measuring the light passing so that fiat sides on the exit ends thereof are in abutting relation so that the `ioint between the two forms a line of demarcation separating the two matching fields which will substantially disappear when the two elds are properly matched in light intensity.

Another object is to provide adensitometer which includes a pivoted head which may be moved away from the base to place a film negative in measuring position, and which may then be dropped down on the film for measuring purposes; and in which all of the parts necessary- And a further oblectis to provide means on' the lower end of the head of the instrument which serves to both protect the nlm and the end of-one of the light-transmitting rods from iniury and to properly position said end of the rod a proper distance from the film surface for measurement.

And another object is to provide alight'trans-V mitting and integrating rod which permits the entrance end of the rod to be reduced to a minimum cross-sectional area so that small areas of theillm may be measured,

through the negative, and showing the' means for protecting the end of said rod and the film and for positioning the end of the rod relative to the film surface, l

Fig.l 6 is an enlarged plan view of the compari- 'son field of the instrument showing how it appears when the two fields are not matched,

Fig. 6 is a view similar to Fig. 5 and showing how the comparison field appears when the two fields are matched in intensity, and

Fig. 7 is an enlarged top view of the window in the base over which the negative to be measured is positioned.`

Like reference characters refer to correspond? 'ing parts throughout the drawings.

Referring now to the drawings, the'present densitometer comprises a hollow base I which may be molded of a suitable plastic material. This base includes a fiat top Ii which terminates in a vertical apron i2 the bottom'edge of which serves to support the instrument o n a table for use. Located substantially in the center of the top Il is a window I3 over which a film nega?v tive F, the density of which is to bel mc'as d is laid in a flat condition, as is well knownand as indicated in Fig. 4; This window ipr'eferably consists of a glass disk which is situ e'din a recess molded into the top vof the b A the top of the disk is ilusliffwith t base. As shown in Fig.7,., the en with the exception of a s l V circle, diri the' center. is rendered light diffusing while the circle in the center is left clear. 'clear circlefy'is approximately the sizeof they area of thel point-.fin

the nlm toy be measured, or approximately'the size of the tapered end on 'the'lig'ht-transinitting rod, to be described. so that it aids the operator in positioning the negative with the area he desires to measure in 'a proper location for measurement. Furthermore. the major portion of the window which is not used during the density measurement. but is necessary to aid in selecting the desired area. would blind the operator if it were clear and not light diffusing.

This window is adapted to be illuminated by a light source in the forin oi' a small low voltage lamp I4 which is adjustably mounted within the base so that the distance between the lamp and the window may be altered to vary the intensity of illumination of the window for the purpose of initially balancing the instrument, as is well known, and as will be pointed .out below. The mounting means for the lamp consists of a pivoted lever or walking beam I5 pivoted intermediate its ends ina yoke I8 fixed to bosses I1 formed on the under side of the top of the base. One end of the lever has a turned-down end I8 onto which isslipped a socket or clip I9 in which the lamp I4 is positioned. The other end of the lever is pivoted at 2li to a block 2I which is lin threaded engagement with an adjusting screw 22 which extends through and is rotatably mounted in the top of the base. The top of this screw may be ush with the top of the base and be provided with a slot for the reception of' a screw-driver, etc., for adjustment, or as shown it may extend above the top of the base and be provided with a knurled hand knob 33. A doubleended coil spring S is mounted on the hinge pintle of the lever between the arms of yoke I6 and serves to normally force the lever to pivot in one direction to eliminate any back lash in the adjusting screw 22. By turning the adjusting screw, the lamp I4 is moved toward or away from the window I3 to vary the intensity of illumination ofthe same, and the lamp mounting means is such that an interchange of lamps, in case of failure. etc.. may be readily effected.

Located on the top of the base is a hollow head shown, since it constitutes no part o! the present invention, but the transformer 34 and the switch 35 are shown in the positions they assume in or on the base. The cord 33 for plugging the device into a wall socket is shown coming out of the top of the base at the rear thereof. If desired, an adjustable two-position rheostat, not shown, might be included in the circuit of one or both lamps to permit the lamps to be dimmed for selection of the area of the negative to be measured and then be turned on bright for the actual density measurement of the selected area.

Rotatably mounted in a vertically disposed semi-circular portion 31 of the head is a neutral density wedge 38. the variable density portion of which passes in front of the lamp 30. While this wedge may be constructed in any well-known manner, I have found that it can be cheaply made by molding from a transparent plastic impregnated with a suitable pigment. such as carbon black. The wedge portion of the disk comprises a circular channel 33, varying regularly and con- 23 which may be readily molded from a plastic material, and which houses the majority of the optical means for forming the two matching fields and the complete comparison eld and getting the iight beams thereto. At its rear end this head is hinged to the rear edge of the top'of the base so that it can be lowered to an operative v. position, in which it is shown, or it can be raised a dim loading, or inoperative, position indited by the dotted line position shown in Fig. 3.

hinge connection comprises a pintle 24 passthrough apertured bosses 25 and 26 formed I on the base and head, respectively, said pintle being held in position by a set screw 21 threaded tinuously in depth throughout its length, formed in one face of the disk. The varying thickness left by this channel forms a very satisfactory density wedge, and the channel may be machined out or it may be molded into the disk if desired.

I'his density wedge is rotatably mounted in the head on a stub shaft 40 fixed to the rear wall of the semi-circular portion 31 by means of a cap nut 4I, and screws 42 to keep it from rotating. The wedge is held against free rotation by a spring washer 43 which frictionally engages one face thereof and forces the other face of the wedge against the ange 44 associated with the stub shaft 40. Fixed to the periphery of the density wedge in any suitable manner, as by a setscrew, not shown, is a knurled rim 45 the face of which includes a scale 43 reading in density values. As shown in Fig. 2, the rim of the disk extends`through a cutaway part 41 of the semi-circular portion31 of the head so that the wedge can be rotated by a person engaging the knurled rim with his linger, and so that the scale 46 co-operates with an index mark 43 on the face of the semi-circular portion 31 of the head to facilitate reading the scale.

The bottom of the forward wall of the semicircular portion 31 of the head terminates in a horizontal wall 43 and a vertical apron .60. A metal tube 5I of the form best shown in Fig. 2

, is fastened to the lower side of this horizontal nio one of the bosses 25 on the base. A coiled spring 28 surrounding the pintle, and having one end engaging the lower side of the head and the other end lying in a recess 29 on the top of the base, serves to counter-balance the weight of the head so that it will not drop freely onto the top of the base when released.

In the rear horizontal portion of the head there is mounted a light source in the form of a lamp wall by screws 52 passing through wings 53 on the tube and into tapped holes in said wail. As clearly shown in Fig. 3, the upper` end 54 of this tube extends upwardly through said horizontal wall, while the lower end 55 thereof extends downwardly just short oi the top of the base when the head is in its operative position. As shown in' Fig. 3, the tube includes a branch 5B which extends toward, and terminates just short of, the density wedge.

Within a recess 51 formed in said tube 5| there are mounted a pair 'of light transmitting and integrating rods 58 and 59 which are adapted to direct the light passing through the negative and the light passing through the wedge to two matching fields which in combination form a comparison field for matching purposes. The rod 58 is straight while the rod 53 is curved, and they are mounted in the recess $1 of the tubeso that the light entrance end 60 of the curved rod 59 is adjacent the density wedge and in alignment with the light rays passing through said wedge, while the light entrance end 3l of the rod A i s Il is adiacent-thelwndow' and in alignment with the clear circleC therein. or in a position to receive the light raya passing through that portion cular in cross section, extending longitudinally of the rod so that they can be brought into sideby-side relation and be firmly held in such a position by a setscrew I4. The other ends of the rods are firmly held in the tube by a curved metal strap l! which is fastened to the branch II of the tube by screws. This strap not only serves to hold the rods Il and Il in position but also acts as a light trap to prevent the light in the lamp I from striking the walls of the rods and aifecting the accuracy of the instrument.

The rods Il and II may be made of any transparent material and I havefcund that a trans- -parent synthetic plastic material, e. g. methyl methacrylate sold under the trade name, Luclte, is particularly suitable because it not only possesses the desirable light transmitting characteristics. but it is cheap, easy to fabricate, and is unbreakable. Rays of light entering one end of these rods are multiply reflected from the -walls thereof, because of the well-known principle of total internal reflection due to thediiference in the refractive index between the transparent material and air, and subsequently pass from and illuminate the other end of the rods. Because of the fact'that practically all of the light rayspassing through the rods are totally reflected when they strike the walls thereof, there is very little loss of light upon passing therethrough and practically all of the light entering'one of the 'rods passes to the other end thereof. Any loss of light which might be encountered will be found in the curved rod only and then at the bend thereof when the angle of incidence of certain rays of the light might drop below that necessary for total reection. If this difnculty is encountered because of the radius of the bend found necessary in the curved rod, thewalls of the curved portion of this rod may be coated with an opaque reflecting material to overcome all loss of light, but I have found that if the curved rod has a curvature of the order of that shown in the drawings that satisfactory results are obtained if the walls are left untreated.

By virtue of the fact that light rays entering one end of the rod are multiply reflected by the walls of the rod during its passage therethrough to the other end. the light entering the rod is integrated while passing' through the rod and is uniformly spread over the area of the exit endI thereof. This factmakes itpossible to usey the exit end of these rods asv the matching nelds of the densitometer and does away with the need optical contact. of course, ecause of the air layer requiredadjacent all walls of the rods for total internal reflection of the light rays. If the ends vlarly difllcult for a person suering a deficiency in co'lor sense. With the present arrangement of parts.'the dimculty of correctly matching the two fields is greatly eliminated because the absolute reliance on the color sense of the operator is eliminated. In the present instrument, theI line 1of Aseparation between the two exit ends of the rods appears as a decidedly dark line in the comparison field when vthe intensities of illumination of the two ends of the rods are not matched, and this line of division gradually reduces in density as the fields approach a match. until when the fields are perfectly matched this line is practi` cally invisible andthe two matching lflelcls appear as one. In Fig. 5 the complete comparison field of the instrument is shown as it would appear when the two matching ilelds are far from matched. It will be noticed that the two fields are separated by a dark line 86 which isthe line of separation between the ends of the rods 58 and 58. In Fig. 6, the comparison field is shown as it would appear when the two fields are perfectly matched, and it will be noticed that the dividing line 88 has disappeared and the matching fields appear as one. While a thin/layer of air between the abutting ends of the rods is preferred to obtain the necessary -total internal reflection at these portions ofthe rods in order'to obtain the separation line 66 which substantially disappears when the fields are matched. a thin layer of any suitable'rellecting material could be applied to one or both of the abutting faces of the rods t obtain the necessary total internal reflection. This procedure is not as satisfactory as the layer of air, however, since it would produce a line of separation between the matching fields which would not disappear when the fields were matched in intensity.

To facilitate viewing the comparison field of the instrument, a light excluding tunnel 81 is fastened on the front wall of the semi-circular for elaborate optical systems including light .lii'- fusing means for the matching nelds as in conventional instruments of this type. Consequently, the exitends of the two rods 58 and 69 are polished andare used as the matching fields of the instrument. Inasmuch as the 4two exit ends l2 and 63 ofthe rods are in abutting relation, the two matching fields will be in side-byside relation for ready and easy comparison. 'As

will undoubtedly be obvious to those skilled in the art, when I refer to the flat sides of the end s I2 and 6I of the rods lbeing in abutting relation, I mean mechanical abutment to provide the line of separation between the matching ilelds but not portion l1 of the head by screws B8. 'Ihe lower area of the .two exit ends ot the rods 58 and '59 framed by the aperture 89 in the tunnel 81 could be used as the comparison field, I have found that matching of the fields is made easier if their areas are reduced. Accordingly, I slip a mask 10 onto the top end M of the tube 5I 'which includes a circular opening 'il and which deflnes a reduced circular comparison field made up of two adjacent semi-circular matching fields, as shown in Fig. 5.

In order to facilitate the density measurement of a small selected area of a tllm negative, the entrance end ofthe rod Bl is tapered down to a point, as shown in Fig. 4, having a cross-sectional area comparable to that area on a film which 7 it is desired to measure and which is many times reduced with respect to the cross-sectional area of the exit end of the rod. I have found that l reducing .the entrance end of this rod to a crosssectional area of a. few square millimeters provides a very selective instrument. As shown in Fig. 4, the tapered end of rod 58 extends below the lower end 55 of tube 5i. Fixed to the lower end 55 of the tube 5I in encricling relation with the tapered end of rod 58 is a washer 12 of resilient material, e. g. rubber or the like. This washer is of such thickness that its lower surface extends slightly beyond the end of the tapered point of rod 58 and serves several purposes. First, it protects the window i3 from being broken if the head is dropped downl thereon too hard. Secondly, it protects the tapered point of the rod 58 from being damaged by striking the window and/or prevents the end of the rod from picking up dirt or dust from the surface of the film or from the surface of the Window. Third, it serves to properly and slightly space the tapered end of the rod 58 from the iilm negative when the head is dropped down on the film, the end of the rod just clearing the nlm, see Fig. e. And last, it prevents the illm surface from being scratched or otherwise damaged by the end of the rod.

The fact that the rods have the property of integrating the light passing therethrough over the entire area of the exit end thereof maires it possible to use an entrance end which is many times smaller than the exit end. The difference in the amount ofiight capable oi entering the two rods t and 5%, because oi the difference-s in cross-sectional area of their entrance ends, can be accounted for in balancing the instrument prior to use, namely, by placing the lamp i4 closer to the end ont rod tti than lamp t@ is relative to the end of rod 59.

I have found that for any given cross-seetional area of transparent synthetic plastic rod that a substantial increase in light transmission from a given light source will be obtained if the rod is made from a plastic material which has been rolled to shape rather than extruded to shape. Just Why the light transmission of a rolled plastic material is greater than the light transmission of the same plastic extruded to shape is not known, but this fact has been proven by tests to be true. Accordingly, if the maximum brilliance of the matching'elds inthe present instrument is desired the rods 58 and 59 should be made from a sheet of plastic material polled to a thickness to obtain one desired dimension and cut oi from the sheet in strips having the width of the other desired dimension. This phenomenon might be utilized to eliminate the difference -in light transmission caused by having the cross-sectional area of. the light entering end of the rod 58 4smaller than that of rod 59. For instance, the rod 5B could be made of a rolled plastic material while .the rod 59 could be made of an extruded plastic material so that the transmission properties of the rod 58 will be increased over that of the rod 59 to compensate for the difference in the quantity of light entering the two rods due to the gifference in the areas of their lightl entering aces,

'I'he operation of the present densitometer might be outlined to bring out the advantages of the same. First of al1, the instrument. musty be balanced. This is done by turning on the window i3 with no film in place, and turning the density wedge until the density scale 48' reads zero. The position ofvlamp I4 is then adjusted toward and from the window I3 by turning screw 22 until the two matching fields formed by the ends of rods 58 and 59 are matched as shown lin Eig.. 6, and as clearly indicated when the dividing line 65 between the elds, see Fig. 5.

.clear circle on an otherwise diffusing window makes this selection very easy and accurate, Then the head is lowered onto the nlm so that the tapered end 6l of rod 5B is directly over the selected area of the negative. Looking into the tunnel 6l at the comparison iield, the density wedge is then rotated until the two semi-circular matching iields are matched as indicated by the disappearance of the dividing line 66 between them, and then the value of the density of the selected area of the negative is'read directly oi of the scale d6.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modiiications thereof are possible. My invention, therefore, is not to be restricted to theprecise details of construction shown and described, but is intended to cover all modiiicatlons coming within the scope of the appended claims.

Having thus described my invention, what I claim is new and desire to secure by Letters Patent of the United States is:

l. A densitometer comprising incombination a support, two matched light sources mounted on said support in spaced relation and shielded from one another; a window disposed in the path of the rays of one of said light sources and over which a photographic negative the density of which is to be measured is adapted to be disposed for illumination.; a density wedge movable across the path of the rays of the other light source; a first rod of light transmitting material on said support for transmitting the light passing through said negative to a matching field; the light entering'end of said rod being disposed adjacent the surface of said negative, the other end of said rod constituting a matching eld and having a plane side extending longitudinally of the rod; a second rod of iight transmitting material on said support for transmitting the light passing through said density wedge to a matching field; the exit end of said rod having a plane side extending longitudinally of the rod, and constituting a second matching eld, `said rods so disposed relative to one another that the plane side of the exit ends thereof are in abutting relaitiozr but separated to prevent optical contact and the exit ends of the two rods constitute the matching nelds and in combination form a comparison field having a line of demarcation which will substantially disappear when the two elds making up the comparison field are matched-in intensity by rotation of said density wedge.

2.- A densitometer according to claim 1 in which the light entering end of said first rod is tapered lights, lowering the head 23 directly onto the 75 toward the light entering end to a, cross section assmn f 9 substantially less than that of themain portion of said rod to facilitate the density measurement anangle to, and adjacent. one another for purposes of compaotness-and said two light transmitting rods are madeiof a transparent plastic material and one is straight while the other is curved to accommodate said disposition of the window and density wedge.

4. A densltometer according to claim 1 in which Y said support comprises a hollow base portion in the top of which said window is located and within which, and below said window, is disposed 'one of said light sources; andl a head pivoted to the top of said base to move to and from the same; said other light. source,- said density wedge and said two light transmitting rods being mounted within saidhead `and so positioned therein that when the head is lowered onto the ton of said base, the light entering end 'of said first rod is disposed above'and in spaced relation to said window.

5. A densitometer according to claim 1 in which said window is provided with 'a closure which is light diifusing with the exception of a small transparent area substantially equal to the area of the light entering end of said first rod and in alignment with posed. .v

which said end oi' the rod is dls- 6. Adensitometer comprising in combination a hollow base; a light diusing window in the! top of said base over which a photographic negativ'e is adapted to -be positioned; a light source within said base below said window for illuminating the same; means for adjusting the position of said light source to and from said window to vary the intensity of illumination of said window; a head pivotally mounted on thetop of said base to move to and from an operative position; a second light source substantially matched with that in the base mounted in said head; a density wedge movably mounted ,in said head inintercepting relation with the light rays of said second v operativeposition. and the entering end Vof the other receives the light rays passing through said density wedge, and so that the exit ends of the rods have their flat sides in abutted relation but free from optical contact, whereby two matching fields are formed in side-byside relation, which are separated by a visible line of demarcation vwhich will substantially disappear when the matching fields are matched in light intensity.

'1. A densitometer according to claim o in which the means for adjusting the position of said light source to and fromsaid window includes a walking beam pivoted in said housing; said light source'flxed to one end of said beam; and an v l0 adjusting screw in threaded ensagement theother end" of said beam and accessible for A adjustment from the top of said base.

8.' A densitometer according to claim 6 in which a scale associated with the movable density 'wedge co-operates with a fixed index on the head to give the values 'of 'density measured on the instrumgnt.

a 9a A densitometer according to claim 8 in which the entering end vof the rod above the window 'is reduced to approximately l sq. mm. in crosssectional area while the cross-sectional area of the exit end of this rod is many tlmesthis value to provide a matching ileld of sumcient size -to be easily judged 1o. A densuometer according to claim e in which the rods are made'from a transparent plastic material which has characteristics resultmoved to its operative'position. the thickness of said washer being such as to just space the endf of the rod from the film and window when the head is moved to its operative position.

12. A densitometer comprising in combination a support: two matched light sources mounted on said support in spaced relation and shielded from one another; a window disposed in the'path of the rays of one of said light sources and over which a photographic negative the"density of which is to be measured is adapted to be disposed for illumination; a density wedge movable across 'the 'path of the rays of the other light source; a

first rod of light vtransmitting material on said support for transmitting the light passing through said negative to a matching iield: the light entering end of said rod being disposed adjacent the surface of the negative. the other end of said rodA constituting a 'matching neld and having a plaiie side extending longitudinally of the rod: a second rod of light transmitting material on said support having one end adjacent the density wedge and for transmitting the light passing through said wedge to the other end of the rod which constitutes avmatching field and which is provided with a plane side eiitending longitudinallyof the rod. said rods so disposed relative to one another that the plane sides of the exit ends thereof are in parallel side-by-side relation and in combinationa constitute a divided comparison field adapted to facilitate balancing of the two matching ilelds; and means for preventing light from passing from one rod to the other at the points where the plane surfaces thereof are in side-,by-side relation. i

13. A densitometer, according to claim 12, in which said last-mentioned means comprises a. thin layer of light-reflecting material separating the plane surfaces of the exit ends of said rods.

CLIFTON .M. TUTILE.

from the negative is 

